Estimate the theoretical maximum efficient speed of a displacement-hull boat, based on the length of its waterline.
How it works
Hull speed uses the classic naval-architecture rule of thumb: Hull Speed (knots) = 1.34 × √(waterline length in feet). A 25-foot waterline gives a hull speed of about 6.7 knots. This coefficient comes from the physics of the bow wave a displacement hull generates as it moves.
What this does not include
This is a classic rule-of-thumb approximation for traditional displacement hulls, not a hard physical limit — modern hull designs and planing hulls can exceed it, and this calculator doesn’t model hull shape, weight, or power beyond the basic waterline-length relationship.
How to use this calculator
- Enter the boat’s waterline length in feet.
- Read off the estimated hull speed in knots.
A worked example
A boat with a 25-foot waterline length: theoretical hull speed = 1.34×√(waterline length) = 6.7 knots.
A 40-foot waterline: hull speed = 8.4749 knots — a much longer boat only gains a modest speed increase, since the relationship follows a square root, not a straight line.
What the variables mean
| Variable | Meaning |
|---|---|
| Waterline length | Length of the hull at the waterline, in feet |
Edge cases worth knowing
Hull speed grows with the square root of length, not proportionally. The 40-foot boat above is 60% longer than the 25-foot one, but its hull speed is only about 26% higher — a longer waterline gives diminishing speed returns.
This applies to displacement hulls, not planing hulls. Boats designed to plane (rise up and skim the surface) can exceed this theoretical limit, which only describes the speed where wave-making resistance becomes dominant for a traditional displacement hull.
Why does a longer waterline mean a faster hull speed?
A longer hull generates a longer bow wave, and the wave’s natural speed increases with its length — a displacement hull is effectively limited by the speed of the wave it creates.
Can a boat go faster than its hull speed?
Yes, especially planing hulls that rise up and skim across the water rather than pushing through it — the 1.34 rule specifically describes traditional displacement hulls.
Is the 1.34 coefficient exact?
No — it’s a widely used approximation; some references use slightly different coefficients depending on hull shape assumptions.
